function btn_move(el, mouseLeft, mouseTop) {
  var leftRnd = (Math.random() - 0.5) * 20;
  var topRnd = (Math.random() - 0.5) * 20;
  var btnLeft = mouseLeft + (leftRnd > 0 ? 100 : -100) + leftRnd;
  var btnTop = mouseTop + (topRnd > 0 ? 30 : -30) + topRnd;
  btnLeft = btnLeft < 100 ? (btnLeft + window.innerWidth - 200) : (btnLeft > window.innerWidth - 100 ? btnLeft - window.innerWidth + 200 : btnLeft);
  btnTop = btnTop < 100 ? (btnTop + window.innerHeight - 200) : (btnTop > window.innerHeight - 100 ? btnTop - window.innerHeight + 200 : btnTop);
  el.style.position = 'fixed';
  el.style.left = btnLeft + 'px';
  el.style.top = btnTop + 'px';
}

function over_btn(e) {
  if (!e) {
    e = window.event;
  }
  btn_move(this, e.clientX, e.clientY);
}

document.getElementById('su').onmouseover = over_btn;